Multi-Fiber Sensing Topology For Subsea Wells

ABSTRACT

A fiber optic sensing (FOS) system and method. The system may include one or more interrogator units and a proximal wavelength division multiplexer (WDM) and a distal WDM optically connectable to the one or more interrogator units, an upgoing transmission fiber, a down-going transmission fiber, and one or more downhole sensing fibers. The method may include transmitting one or more light pulses from an interrogator unit, multiplexing the one or more light pulses from the interrogator unit with a proximal WDM into an upgoing transmission fiber and a down-going transmission fiber, and receiving the one or more light pulses with a distal WDM. The method may further include multiplexing the one or more light pulses from the upgoing transmission fiber and the down-going transmission fiber into one or more downhole sensing fibers and receiving backscatter light from at least one of the one or more downhole sensing fibers.

BACKGROUND

Boreholes drilled into subterranean formations may enable recovery ofdesirable fluids (e.g., hydrocarbons), or geological storage of otherfluids (e.g., carbon dioxide), using a number of different techniques. Anumber of fiber optic sensing (FOS) systems and techniques may beemployed in subterranean operations to characterize and monitor boreholeand/or formation properties. For example, Distributed TemperatureSensing (DTS) and Distributed Acoustic Sensing (DAS) along with a fiberoptic system may be utilized together to determine borehole and/orformation properties including but not limited to production profiling,solids production, injection profiling, flow assurance, vertical seismicprofiling, well integrity, geological integrity, and leak detection.Distributed fiber optic sensing is a cost-effective method of obtainingreal-time, high-resolution, highly accurate temperature and/or strain(static or dynamic, including acoustic) and/or pressure data along theentire wellbore. Discrete (or point) fiber optic sensing, e.g., by usingfiber Bragg gratings (FBGs), is an alternative cost-effective method ofobtaining real-time, high resolution, highly accurate temperature and/orstrain data at discrete locations along the wellbore. Moreover, FBGs andthe downhole cable may be integrated with transducers capable ofinducing temperature and/or strain upon at least one FBG, thus providingan optically proportional measure of transduction, e.g., for sensingpressure, voltage, current, or chemical concentration. Additionally,fiber optic sensing may eliminate downhole electronic complexity byshifting all electrical and electro-optical systems to the surfacewithin the interrogator unit(s). Fiber optic cables may be permanentlydeployed downhole in a wellbore via single- or dual-trip completionstrings, behind casing, on tubing, or in pumped down installations; ortemporally via coiled tubing, wireline, slickline, or disposable cables.

Distributed fiber optic sensing can be enabled by continuously sensingalong the length of the optical fiber, and effectively assigningdiscrete measurements to a position or set of positions along the lengthof the fiber via optical time-domain reflectometry (OTDR). That is, byknowing the velocity of light in fiber, and by measuring the time ittakes the backscattered light to return to the detector inside theinterrogator, it is possible to assign a measurement and distance alongthe fiber. In alternative embodiments, functionally equivalentdistributed fiber optic sensing data may be acquired via opticalfrequency-domain reflectometry (OFDR) techniques.

DAS, DTS, and FBG sensing has been practiced for monitoring downholesensing fibers in dry Christmas tree (or dry-tree) wells to enableinterventionless, time-lapse temperature, acoustic, and pressuremonitoring borehole and/or formation properties including but notlimited to production profiling, solids production, injection profiling,flow assurance, vertical seismic profiling, well integrity, geologicalintegrity, and leak detection. For installation in dry-tree wells,multiple sensing fibers are typically integrated in a tubingencapsulated fiber (TEF) cable. This enables, for example, a DAS systemto preferentially sense a single-mode downhole sensing fiber, and a DTSsystem to preferentially sense a multi-mode downhole sensing fiber; suchthat the DAS and DTS systems are operated simultaneously but are notsimultaneously sensing the same downhole sensing fiber. Typically, theinterrogator units are adjacent to, or a short distance, from the wellhead outlet on the dry Christmas tree.

For downhole sensing fibers installed in subsea wells, marinization ofthe interrogator(s) (i.e., packaging interrogators for deployment on astructure residing on the sea floor proximal to a subsea Christmas tree)introduces significant complexity and cost to the Subsea ProductionSystem (SPS) and related electrical and optical distribution systems anddoes not readily permit interrogator hardware upgrades. It is preferableto maintain any interrogator system(s) on the topside facility, and tosense the downhole sensing fiber through optical distribution in thesubsea infrastructure. However, such a subsea well sensing operationthen requires optical engineering solutions to compensate for insertionlosses accumulated through long (˜5 to 100+km) lengths of subseatransmission fiber between the topside facility and subsea tree (e.g.,static umbilical lines, dynamic umbilical lines, jumper cables, opticalflying leads), up to 10 km of downhole sensing fiber, and multiple wet-and dry-mate optical connectors, splices, and an optical feedthroughsystems (OFS) in the subsea Christmas tree (XT).

The current (horizontal or vertical) subsea XTOFS by TE Connectivityenables optical wet-mating of a single fiber when the XT is landed onthe tubing hanger. Thus, the number of downhole sensing fibers in asubsea well is currently limited to one. However, multi-fiber OFSs arebeing developed, and will enable multiple (e.g., three to six) downholesensing fibers in a subsea well. Whether one or more downhole sensingfibers are installed in the subsea well, they require optical continuityback to the topsides facility so the backscattered light can be receivedby the interrogator(s). The optical connectors used in the subseainfrastructure (e.g., at umbilical termination assemblies, opticaldistribution units, drill centers, optical flying leads, and subseatrees) are finite in their optical circuit (or pin) count. For example,current wet-mate connector technology may support eight, twelve, ortwenty-four pins. Also, there are physical limits (i.e., real estate) asto how many connector receptacles may be installed on subsea equipment.For example, umbilical termination assemblies (UTAs) may terminatemultiple electric, hydraulic, and fiber optic lines through a finitenumber of electrical, hydraulic, and optical connector receptacles bemounted on their remotely operated vehicle (ROV) panels. Thus, withoutan efficient method to manage optical distribution, there isconsiderable complexity and cost when contemplating the use of multipledownhole sensing fibers in multiple subsea wells.

While some fiber optic sensing techniques such as DAS and DTS may beoperated simultaneously or sequentially or periodically on the samedownhole sensing fiber, having additional downhole sensing fibers mayallow for other simultaneous, sequential, or periodic sensingapplications, such as arrays of discrete pressure and temperature gaugesvia an FBG system. Currently, there exist an inability to maximize thenumber of downhole sensing fibers in a wellbore while minimizing thenumber of subsea transmission fibers between the interrogator(s) locatedat the topside facility and the OFS.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred examples of the disclosure,reference will now be made to the accompanying drawings in which:

FIGS. 1A and 1B illustrate an example of a well measurement system in asubsea environment;

FIGS. 2A-2C illustrates examples of a downhole fiber deployed in awellbore;

FIG. 3 illustrates an optical distribution unit;

FIG. 4 illustrates an umbilical termination assembly;

FIG. 5 illustrates an optical flying lead;

FIG. 6A illustrates an optical feedthrough system;

FIG. 6B illustrates a cutaway of at least a part of subsea tree;

FIG. 7 illustrates an example of a FOS system;

FIG. 8 illustrate an example of a FOS system with lead lines;

FIG. 9 illustrates a schematic of another example FOS system;

FIG. 10 illustrates an example of a remote circulator arrangement;

FIG. 11 illustrates a graph for determining time for a light pulse totravel in a fiber optic cable;

FIG. 12 illustrates another graph for determining time for a light pulseto travel in a fiber optic cable;

FIG. 13 illustrates an example of a remote circulator arrangement;

FIG. 14 illustrates another graph for determining time for a light pulseto travel in a fiber optic cable;

FIG. 15A illustrates a graph of sensing regions in the DAS system;

FIG. 15B illustrates a graph with an active proximal circulator using anoptimized FOS sampling frequency of 12.5 kHz;

FIG. 15C illustrates a graph with a passive proximal circulator using anoptimized FOS sampling frequency of 12.5 kHz;

FIG. 16 illustrates a graph of optimized sampling frequencies in the FOSsystem;

FIG. 17 illustrates an example of a workflow for optimizing the samplingfrequencies of the DAS system; and

FIGS. 18-24 illustrate other examples of the FOS system;

FIG. 25 illustrates an example of the FOS system in a land based well.

DETAILED DESCRIPTION

The present disclosure relates generally to a system and method forfiber optic sensing, which may include but not limited to Fiber BraggGratings (FBGs), Distributed Acoustic Sensing (DAS), DistributedTemperature Sensing (DTS), Distributed Strain Sensing (DSS), DistributedChemical Sensing (DCS), Distributed Magnetomotive Force Sensing (DMS),Distributed Electromotive Force Sensing (DES), and DistributedBrillouin-Frequency Sensing (DBFS), the latter which may be used in theextraction of distributed strain, temperature, or pressure or acombination thereof. It should be noted that any, or any combination ofall systems and methods described above are generally referred to as aFiber Optic Sensing (FOS) system. Subsea well sensing operations maypresent optical challenges which may relate to the signal fidelity andquality of FOS system given the long transmission fiber and multipleoptical connections required to lead into the downhole sensing fibers.The sensing region of interest is typically the downhole sensing fiber(i.e., the in-well and reservoir sections), and not the transmissionfibers (i.e., OFLs, jumpers, and static and/or dynamic umbilical lines).

To prevent a reduction in FOS signal-to-noise (SNR) and signal qualityand fidelity, the FOS system described below may increase the returnedsignal strength with given pulse power for emitted light, decrease thenoise floor of the receiving optics to detect weaker power pulses,maintain the pulse power as high as possible as it propagates along thetransmission fiber(s), increase the number of light pulses that may belaunched into the downhole sensing fiber(s) per second, and/or increasethe maximum pulse power that may be used for given fiber length.

FOS systems utilize one or more downhole sensing fibers integrated infiber optic cables (or tubing encapsulated fibers, TEFs). One or moreelectrical conductors may be integrated in the TEF so as to provideelectrical power and/or telemetry to a downhole device, e.g., a pressuregauge. Downhole sensing fibers may be at least one single-mode fiber(SMF), at least one multi-mode fibers (MMF), or a combination of atleast one SMF and at least one MMF. Each of the at least one SMF or MMFmay be treated with a coating to prevent undesirable effects, e.g.,hermetically sealed in carbon to delay hydrogen degradation. Each of atleast one SMF or MMF may be treated with a coating to generate desirableeffects, e.g., induced strain via improved strain transduction, achemical reaction, or exposure to an electromotive or magnetomotiveforce. At least one SMF may further be enhanced (or engineered) to yielda higher-than-Rayleigh scattering coefficient so as to increase the DASsignal to noise ratio (SNR) by 10 dB to 20 dB. Such enhanced backscatterfibers (EBF) may consist of either weak, distributed gratings, ordiscrete gratings in a SMF. The EBF may be fabricated with a narrowenhanced backscatter bandwidth, such that a DAS system may be sensitiveto the enhanced backscatter, but at least one other FOS system does notexhibit any appreciable sensitivity to the enhanced backscatter than itwould if sensing a standard (or non-enhanced) SMF. The EBF may befabricated with a broad enhanced bandwidth, such that a DAS system andat least one other FOS system may exhibit sensitivity to the enhancedbackscatter.

Fiber optic cables may be permanently deployed in a subsea well viasingle- or dual-trip completions. Fiber optic cables may include one ofat least one optical fiber encapsulated in a hydrogen-scavenginggel-filled stainless steel tube and may further be encapsulated in ametallic (e.g., Inconel® alloy 825) armor. A hydrogen delay barrier maybe located between the stainless-steel tube and the armor, e.g., ametallurgical hydrogen delay barrier such as aluminum may be extrudedupon the stainless-steel tube before encapsulation in the metallicarmor. The fiber optic cables may be further encapsulated in athermoplastic encapsulation.

FOS systems utilize transmission fibers integrated in the subseainfrastructure fiber optic cables to provide optical continuity betweenthe interrogator(s) located at the topside facility and downhole sensingfiber(s) in the subsea well. The transmission fibers may be integratedwithin OFLs, jumpers, and static and/or dynamic umbilical lines, andoptically coupled via splices, wet-mate connectors, and/or dry-mateconnectors. Transmission fibers may be either SMF or MMF. In someembodiments, the transmission fibers may be low-loss (LL) or ultra-lowloss (ULL) SMFs that have lower optical attenuation and higher powerhandling capability before non-linearity so as to enable high gain, co-or counter-propagating distributed Raman amplification. For example,pure silica core SMF, such as Corning® SMF-28® ULL SMF, typicallyexhibit 0.15 to 0.17 dB/km optical attenuation at 1550 nm wavelengths.

FOS systems may employ distributed fiber optic sensing, which is acost-effective method of obtaining real-time, high-resolution, highlyaccurate temperature, strain, and acoustic/vibration data along theentire downhole fiber, while simultaneously eliminating downholeelectronic complexity by shifting all electro-optical system complexityto the interrogator unit(s) located at the topside facility. Example ofdistributed fiber optic sensing include distributed acoustic sensing(DAS), also referred to as distributed vibration sensing (DVS), whichpreferentially operates with SMF; distributed Brillouin-frequencysensing for distributed temperature and/or strain sensing and/orpressure sensing (DTS/DSS/DPS) preferentially operates with SMF; andRaman DTS which preferentially operates with MMF. Other distributedfiber optic sensing may include but not be limited to distributedchemical sensing (DCS), distributed electromotive force sensing (DES),and distributed magnetomotive force sensing (DMS).

Distributed fiber optic sensing may operate by continuously sensingalong the length of the downhole sensing fiber, and effectivelyassigning discrete measurements to a position along the length of thefiber via optical time-domain reflectometry (OTDR). That is, by knowingthe velocity of light in fiber, and by measuring the time it takes thebackscattered light to return to the detector inside the interrogator,it is possible to assign a distance along the fiber. In alternativeembodiments, functionally equivalent distributed fiber optic sensingdata may be acquired via optical frequency-domain reflectometry (OFDR)techniques.

Discrete, or point, fiber optic sensing is an alternative cost-effectivemethod of obtaining real-time, high-resolution, highly accuratetemperature and/or strain (acoustic) data at discrete locations/pointsalong the entire wellbore, while simultaneously eliminating downholeelectronic complexity by shifting all electro-optical complexity to theinterrogator unit(s) located at the topside facility. Point sensors mayinclude one or more fiber Bragg gratings (FBGs), where the opticalwaveguide containing the FBG may be modified by a sensor assembly whichefficiently transduces a measurement to temperature and/or strain uponat least one FBG. An example of such a sensor assembly is a pressure andtemperature gauge, a chemical sensor, and a voltage sensor. FBGs mayoperate with either SMF or MMF.

The subsea well's downhole sensing fiber connects to the subsea opticaldistribution system via an optical feedthrough system (OFS) in thesubsea Christmas tree (XT) and tubing hanger. The XT may be either avertical (VXT) or a horizontal XT (HXT) design, or any hybrid orsimplified solution where to hang off the downhole completions. Themethods and systems described below are agnostic to the use of VXTs orHXTs. In the following description, VXT, HXT, subsea Christmas tree, wetChristmas tree, wet-tree, and subsea tree are all synonymous. The OFSprovides optical continuity from transmission fibers in the subseaoptical distribution system to the downhole sensing fiber via anassembly of wet- and dry-mate optical connectors and/or splices. Whenthe XT is landed on the tubing hanger, the OFS enables at least onefiber to be optically continuous between the XT's ROV panel and thetubing hanger. Current and future OFS products from TE Connectivity andTeledyne enable at most one, three, or six fibers to be fed through theXT. Fibers may be SMF, MMF, or any combination of SMF and MMF.

From a downhole monitoring system consideration, multiple downholefibers may increase data acquisition opportunities while simplifyingoverall downhole monitoring system complexity. For example, one SMF maybe used for acquiring DAS and/or DTS, and two SMFs may each or both beused for FBG sensing arrays of pressure and temperature gauges. Forintelligent completions, this may potentially eliminate the necessity ofelectric pressure and temperature gauge arrays, and thus simplify subseacontrol and power distribution systems. The challenge is that havingmultiple downhole sensing fibers with their necessity for opticalcontinuity back to the interrogators located at the topside facility,which could place significant complexity, burden, and cost on the subseaoptical distribution system. On a per-well basis, the systems andmethods described below may maximize the number of downhole sensingfibers while minimizing the number of subsea transmission fibers neededfor their continuity from XT to the topside facility.

The subsea optical distribution system provides optical continuity fromthe downhole sensing fiber to the interrogator located at the topsidefacility. The optical distribution system may be stand-alone (separated)or integrated with other (e.g., electric and/or hydraulic) utilities ofthe subsea production system (SPS). This may involve multiple opticalflying leads (OFLs), jumper cables, static umbilical lines, dynamicumbilical lines, subsea umbilical termination assemblies (SUTAs),topside umbilical termination assemblies (TUTAs), surface cables betweenthe TUTAs and interrogator(s), optical distribution units (ODUs), andoptical distribution through drill centers, manifold centers, or othersubsea equipment.

FIGS. 1A and 1B illustrates an example of a well system 100 that mayemploy the principles of the present disclosure. More particularly, wellsystem 100 may include a floating vessel 102 centered over asubterranean hydrocarbon bearing formation 104 located below a sea floor106. As illustrated, floating vessel 102 is depicted as an offshore,semi-submersible oil and gas drilling platform, but could alternativelyinclude any other type of floating vessel such as, but not limited to, adrill ship, a pipe-laying ship, a tension-leg platforms (TLPs), a sparplatform, a production platform, a floating production, storage, andoffloading (FPSO) vessel, a floating production unit (FPU), and/or thelike. Additionally, and without loss of generality, the methods andsystems described below may also be utilized for subsea tie-backs to afixed offshore platform, an onshore facility, or a facility on anartificial island. Moreover, the systems and methods of the presentdisclosure are applicable to onshore reservoirs and related theirfacilities. A subsea conduit or riser 108 extends from a deck 110 offloating vessel 102 to sea floor 106 and may connect to a productionmanifold 112. As illustrated, static pipe 114 may run from productionmanifold 112 to a pipeline end termination 116. Flexible pipe 118 mayattach a subsea tree 120 to pipeline end termination 116. In examples,flexible pipe 118 may travers from production manifold 112 and connectdirectly to subsea tree 120. Additionally, flexible pipe 118 may connectone subsea tree 120 to another subsea tree 120, effectively tying one ormore subsea trees 120 together and allow for a single flexible pipe 118to connect one or more subsea trees 120 to a single production manifold112.

Subsea tree 120 may cap a wellbore 122 that has been drilled intoformation 104. Within wellbore may be a completion system consisting ofone or more tubulars 124 that are connected to subsea tree 120. Duringoperations, formation fluids may be produced from formation 104, andflow through one or more tubulars 124 to subsea tree 120. As subsea tree120 is attached to floating vessel 102, formation fluid may flow fromsubsea tree 120, through flexible pipe 118, pipeline end termination116, static pipe 114, production manifold 112, and up through riser 108to floating vessel 102 for processing, storage, and subsequentoffloading or export.

To monitor downhole operations, a Fiber Optic Sensing (FOS) system 126may be employed from floating vessel 102. FOS 126 system utilizesdistributed and/or discrete fiber optic sensing as a cost-effectivemethod of obtaining real-time, high-resolution, highly accurate physicalmeasurements, such as but not limited to temperature, strain, andacoustic measurements along the entire wellbore, while simultaneouslyeliminating downhole electronic complexity by shifting allelectro-optical complexity to the interrogator unit (IU), also called aninterrogator, located onboard the floating vessel 102. FOS system 126may include an interrogator 128, umbilical line 130, and at least onedownhole sensing fiber 132. As illustrated, interrogator 128 may be atleast partially disposed on floating vessel 102. Interrogator unit 128may connect to umbilical line 130. Umbilical line 130 may include one ormore optical fibers that traverse from a local electronics room (LER) orcentral control room (CCR) to a topside umbilical termination assembly(TUTA) onboard floating vessel 102. Umbilical line 130 may include adynamic umbilical line 134, a subsea umbilical termination assembly(SUTA) 140, and a static umbilical line 136. Umbilical line may furtherinclude optical flying lead 142 and optical feedthrough system 144.Umbilical line 130 may include one or more fiber optic cables. Eachfiber optic cable may include one or more optical fibers.

FIG. 3 illustrates an optical distribution unit 138. As illustrated, oneof ordinary skill in the art may recognize that optical distributionunit 138 may be constructed to withstand pressures, temperatures, and asubsea environment in which optical distribution unit 138 may operateand function. During operations, a remotely operated vehicle (ROV) (notillustrated) may be deployed from vessel 102 or another vessel withoptical distribution unit 138. The ROV may place optical distributionunit 138 in a previously designated area on sea floor 106. Oncedeployed, optical distribution unit 138 may act as a terminal in whichdynamic umbilical line 134 of umbilical line 130 attaches to from vessel102 (e.g., referring to FIGS. 1A and 1B). One or more ROVs may beutilized to attach dynamic umbilical line 134 and static umbilical line136 to optical distribution unit 138. Additionally, this procedure, insome operations, may be performed at the surface on vessel 102. Inexamples, one or more dynamic umbilical lines 134 may attach to one ormore input connectors 300. This may allow for one or more staticumbilical lines 136 to connect to one or more output connectors 302.Thus, one or more static umbilical lines 136 may allow for a singlevessel 102 to service one or more subsea trees 120 that are connected tooptical distribution unit 138. To reach subsea trees 120, one or morestatic umbilical lines 136 traverse to one or more umbilical terminationassemblies 140. Additionally, in examples, a flying optical lead 142(discussed below) may be utilized to connect optical distribution unit138 to one or more subsea trees 120.

FIG. 4 illustrates an umbilical termination assembly 140. Asillustrated, one of ordinary skill in the art may recognize thatumbilical termination assembly 140 may be constructed to withstandpressures, temperatures, and a subsea environment in which umbilicaltermination assembly 140 may operate and function. During operations,one or more ROVs (not illustrated) may be deployed from vessel 102 oranother vessel with umbilical termination assembly 140. The ROV mayplace umbilical termination assembly 140 in a previously designated areaon sea floor 106. Once deployed, umbilical termination assembly 140 mayact as a terminal in which static umbilical line 136 of umbilical line130 attaches to from optical distribution unit 138 (e.g., referring toFIGS. 1A and 1B). One or more ROVs may be utilized to attach staticumbilical line 136 to umbilical termination assembly 140. Additionally,this procedure, in some operations, may be performed at the surface onvessel 102. In examples, one or more dynamic umbilical lines 134 mayattach to one or more input connectors 300. From umbilical terminationassembly 140, an optical flying lead 142 may connect umbilicaltermination assembly 140 at one or more output connectors 302 to anoptical feedthrough system 144 that is disposed in or is at least a partof subsea tree 120 (e.g., referring to FIGS. 1A and 1B).

FIG. 5 illustrates an optical flying lead. An optical flying lead 142 isa flexible connection that may attach optical distribution unit 138 orumbilical termination assembly 140 or any other suitable location in theoptical distribution system to optical feedthrough system 144. Asillustrated, optical flying lead 142 includes a flexible hose 500terminated at both ends with optical wet-mate connectors 504. Flexiblehose 500 includes one or more optical fibers that provide opticalcontinuity between the two optical wet-mate connectors 504. Flexiblehose 500 may be filled with fluid for pressure balancing in subseaenvironments. Additionally, an integrated compartment 502 may bedisposed at any distance along the flexible hose 500. Integratedcompartment 502 may include any number of optical devices, which isdiscussed in detail below. Integrated compartment 502 may be rated as aone atmosphere (1 atm) pressure canister qualified for deployment insubsea environments and may contain a nitrogen-purged atmosphericenvironment. Each optical wet-mate connection 504 is configured to allowfor an ROV to attach optical flying lead 142 to optical feedthroughsystem 144 and optical distribution unit 138 or umbilical terminationassembly 140 or any other suitable location in the optical distributionsystem, as is readily understood to those of ordinary skill in the art.

FIG. 6A illustrates a subsea tree 120 with optical feedthrough system144. As illustrated, one of ordinary skill in the art may recognize thatsubsea tree 120 with optical feedthrough system 144 may be constructedto withstand pressures, temperatures, and a subsea environment in whichsubsea tree 120 and optical feedthrough system 144 may operate andfunction. During manufacturing of subsea tree 120, optical feedthroughsystem 144 may be integrated into subsea tree 120 and tubing hangerassemblies. Subsea tree 120 and tubing hanger assemblies each contain anoptical wet-mate receptacle 600 (e.g., referring to FIG. 6B) that may beoptically coupled when subsea tree 120 and tubing hangers areoperationally deployed. During installation operations, the tubinghanger assembly is coupled to the upper completion of wellbore 124 withoptical continuity to downhole sensing fiber 132 (e.g., referring toFIGS. 1A and 1B), and landed into wellbore 124 on sea floor 106 (e.g.,referring to FIGS. 1A and 2B). Subsea tree 120 is then landed upon thetubing hanger such that subsea tree 120 and tubing hanger are opticallycoupled via the mated optical wet-mate receptacle 600. One or more ROVsmay be utilized to attach optical flying lead 142 (e.g., referring toFIGS. 1A and 1B) to optical wet-mate receptacle 602 located on the ROVpanel 604 of subsea tree 120 and optical feedthrough system 144 as wellas optical distribution unit 138 or umbilical termination assembly 140.In examples, one or more static umbilical lines 136 may attach directlyto subsea trees 120 without optical flying lead 142. Subsea tree 120 andoptical feedthrough system 144 may allow for optical flying lead 142and/or one or more static umbilical lines 136 to connect to one or moredownhole sensing fibers 132.

FIG. 6B illustrates optical feedthrough system 144 formed when thesubsea tree 120 (e.g., referring to FIG. 6A) has been landed upon atubing hanger. In examples, optical flying lead 142 may attach opticalwet-mate receptacle 602 located on ROV panel 604 of subsea tree 120(e.g., referring to FIGS. 6A), which is connected to apressure-compensated flexible hose 606 that terminates with a an opticaldry-mate connection 610 at subsea tree block 608. Optical dry-mateconnection 600 is connected to the subsea tree's optical wet-matereceptacle 600. During installation operations, subsea tree 120 islanded upon the tubing hanger such that subsea tree's optical wet-matereceptacle 600 optically connects to tubing hanger's optical wet-matereceptacle 612. In some embodiments, the tubing hanger's opticalwet-mate receptacle 612 is connected to an optical dry-mate receptacle614 at the base of the tubing hanger, and optically connected to apigtail 618 with optical dry-mate receptacle 616. Pigtail 618 isconnected to downhole sensing fiber 132 via a splice assembly 620 in theupper completion. In other embodiments, tubing hanger's optical wet-matereceptacle 612 is optically connected to downhole sensing fiber 132 viaa splice assembly 620 in the upper completion. Although not illustrated,one or more downhole sensing fibers 132 may be disposed in a fiber opticcable that is optically connected to tubing hanger's optical wet-matereceptacle 612. In examples, an integrated compartment 502 may beinstalled along flexible hose 606 between subsea tree's ROV panel 604and the optical dry-mate connection 600 at subsea tree block 608. Thisintegrated compartment may include any number of optical devices, whichis discussed in detail below. Integrated compartment 502 may be a oneatmosphere (1 atm) pressure canister rated for deployment in subseaenvironments and may contain a nitrogen-purged atmospheric environment.As illustrated, and discussed below in further detail, opticalfeedthrough system 144 allows for optical coupling between opticalflying lead 142 and one or more downhole sensing fibers 132 through asingle connection. As will be discussed in more detail below, downholesensing fibers 132 may allow for downhole measurements to be takenwithin wellbore 122 utilizing principles and function associated withFOS 126.

Referring back to FIGS. 1A and 1B, wellbore 122 extends through thevarious earth strata toward the subterranean hydrocarbon bearingformation 104 and tubular 124 may be extended within wellbore 122. Eventhough FIGS. 1A and 1B depict a vertical wellbore 122, it should beunderstood by those skilled in the art that the methods and systemsdescribed are equally well suited for use in horizontal or deviatedwellbores. During drilling operations, a drill sting, may include abottom hole assembly (BHA) that includes a drill bit and a downholedrilling motor, also referred to as a positive displacement motor(“PDM”) or “mud motor.” During production operations, the completionsystem represented by tubular 124 may include one or more downholesensing fibers 132 of a FOS system 126.

Downhole sensing fiber 132 may be permanently deployed in a wellbore viasingle- or dual-trip completion systems, behind casing, on tubing, or inpumped down installations. FIGS. 2A-2C illustrate examples of differenttypes of deployment of downhole sensing fiber 132 in wellbore 122 (e.g.,referring to FIGS. 1A and 1B). As illustrated in FIG. 2A, wellbore 122deployed in formation 104 may include surface casing 200 in whichproduction casing 202 may be deployed. Additionally, production tubing204 may be deployed within production casing 202. In this example,downhole sensing fiber 132 may be permanently deployed in a completionsystem. In examples, downhole sensing fiber 132 is attached to theoutside of production tubing 204 by one or more cross-couplingprotectors 210. Without limitation, cross-coupling protectors 210 may beevenly spaced and may be disposed on every other joint of productiontubing 204. Further illustrated, downhole sensing fiber 132 may becoupled to a fiber connection 206. Without limitation, fiber connection206 may attach downhole sensing fiber 132 to optical feedthrough system144, and/or umbilical line 130 (e.g., referring to FIGS. 1A and 1B) inthe manner, systems, and/or methods described above. In examples,downhole sensing fiber 132 may further be optically connected toumbilical line 130 through optical flying lead 142 (e.g., referring toFIGS. 1A and 1B). Fiber connection 206 may operate as an opticalfeedthrough system 144 (itself comprising a series of wet- and dry-mateoptical connectors and splices) in the wellhead that optically connectsdownhole sensing fiber 132 from the tubing hanger to umbilical line 130on the subsea tree's ROV panel 604 (e.g., referring to FIGS. 6A and 6B).Umbilical line 130 may include to an optical flying lead 142 and mayfurther include an optical distribution system(s) 138, umbilicaltermination unit(s) 140, and transmission fibers encapsulated in flyingoptical leads 142, flow lines, rigid risers, flexible risers, and/or oneor more static and/or dynamic umbilical lines. This may allow forumbilical line 130 to connect and disconnect from downhole sensing fiber132 while preserving optical continuity between the umbilical line 130and the downhole sensing fiber 132.

FIG. 2B illustrates an example of permanent deployment of downholesensing fiber 132. As illustrated in wellbore 122 deployed in formation104 may include surface casing 200 in which production casing 202 may bedeployed. Additionally, production tubing 204 may be deployed withinproduction casing 202. In examples, downhole sensing fiber 132 isattached to the outside of production casing 202 by one or morecross-coupling protectors 210. Without limitation, cross-couplingprotectors 210 may be evenly spaced and may be disposed on every otherjoint of production tubing 204. downhole sensing fiber 132

FIG. 2C illustrates an example of a pump-down fiber operation in whichdownhole sensing fiber 132 may be deployed either permanently ortemporarily. As illustrated in FIG. 2C, wellbore 122 deployed information 104 may include surface casing 200 in which production casing202 may be deployed. Additionally, capillary tubing 212 may be deployedwithin the production casing 202. In this example, downhole sensingfiber 132 may be permanently or temporarily deployed via a pumpingoperation into the capillary tube.

Referring back to FIGS. 1A and 1B, interrogator unit 128 may beconnected to an information handling system 146 through connection 148,which may be wired and/or wireless. It should be noted that bothinformation handling system 146 and interrogator unit 128 are disposedon floating vessel 102. Both systems and methods of the presentdisclosure may be implemented, at least in part, with informationhandling system 146. Information handling system 146 may include anyinstrumentality or aggregate of instrumentalities operable to compute,estimate, classify, process, transmit, receive, retrieve, originate,switch, store, display, manifest, detect, record, reproduce, handle, orutilize any form of information, intelligence, or data for business,scientific, control, or other purposes. For example, an informationhandling system 146 may be a processing unit 150, a network storagedevice, or any other suitable device and may vary in size, shape,performance, functionality, and price. Information handling system 146may include random access memory (RAM), one or more processing resourcessuch as a central processing unit (CPU) or hardware or software controllogic, ROM, and/or other types of nonvolatile memory. Additionalcomponents of the information handling system 146 may include one ormore disk drives, one or more network ports for communication withexternal devices as well as an input device 152 (e.g., keyboard, mouse,etc.) and video display 154. Information handling system 146 may alsoinclude one or more buses operable to transmit communications betweenthe various hardware components.

Alternatively, systems and methods of the present disclosure may beimplemented, at least in part, with non-transitory computer-readablemedia 156. Non-transitory computer-readable media 156 may include anyinstrumentality or aggregation of instrumentalities that may retain dataand/or instructions for a period of time. Non-transitorycomputer-readable media 156 may include, for example, storage media suchas a direct access storage device (e.g., a hard disk drive or floppydisk drive), a sequential access storage device (e.g., a tape diskdrive), compact disk, CD-ROM, DVD, RAM, ROM, electrically erasableprogrammable read-only memory (EEPROM), and/or flash memory; as well ascommunications media such as wires, optical fibers, microwaves, radiowaves, and other electromagnetic and/or optical carriers; and/or anycombination of the foregoing.

Production operations in a subsea environment may present opticalchallenges for a DAS based FOS system 126. For example, in a DAS system,a maximum pulse power that may be used is approximately inverselyproportional to fiber length due to optical non-linearities in thefiber. Therefore, the quality of the overall signal is poorer with alonger fiber than a shorter fiber. This may impact any FOS system 126that may utilize DAS, since the distal end of the downhole sensing fiber132 may include an interval of interest (i.e., the reservoir) in whichthe downhole sensing fiber 132 may be deployed. The interval of interestmay include wellbore 122 and formation 104. For pulsed DAS systems, inFOS system 126, such as the one exemplified in FIG. 7 , an additionalchallenge is the drop-in signal to noise ratio (SNR) and spectralbandwidth associated with the decrease in the number of light pulsesthat may be launched into the fiber per second (i.e., DAS pulserepetition rate) when interrogating fibers with overall lengthsexceeding 10 km. As such, utilizing DAS system in FOS system 126 in asubsea environment may have to increase the returned signal strengthwith given pulse power, increase the maximum pulse power that may beused for given fiber optic cable length, maintain the pulse power ashigh as possible as it propagates down the fiber optic cable length, andincrease the number of light pulses that may be launched into the fiberoptic cable per second.

FIG. 7 illustrates an example of DAS system for FOS 126. The DAS systemmay include information handling system 146 that is communicativelycoupled to interrogator unit 128. Without limitation, DAS system mayinclude a coherent Rayleigh scattering system with a compensatinginterferometer. In examples, the DAS system may be used forphase-sensitive sensing of events in a wellbore using measurements ofcoherent Rayleigh backscatter and/or may interrogate a downhole sensingfiber containing an array of partial reflectors, for example, fiberBragg gratings.

As illustrated in FIG. 7 , interrogator unit 128 may include a pulsegenerator 700 coupled to a first coupler 702 using an optical fiber 704.Pulse generator 700 may be a laser, or a laser connected to at least oneamplitude modulator, or a laser connected to at least one switchingamplifier, i.e., semiconductor optical amplifier (SOA). First coupler702 may be a traditional fused type fiber optic splitter, a circulator,a PLC fiber optic splitter, or any other type of splitter known to thosewith ordinary skill in the art. Pulse generator 700 may be coupled tooptical gain elements (not shown) to amplify pulses generated therefrom.Example optical gain elements include, but are not limited to, ErbiumDoped Fiber Amplifiers (EDFAs) or Semiconductor Optical Amplifiers(SOAs).

FOS system 126, which is a DAS system, may include an interferometer706. Without limitations, interferometer 706 may include a Mach-Zehnderinterferometer. For example, a Michelson interferometer or any othertype of interferometer 706 may also be used without departing from thescope of the present disclosure. Interferometer 706 may include a topinterferometer arm 708, a bottom interferometer arm 710, and a gauge 712positioned on bottom interferometer arm 710. Interferometer 706 may becoupled to first coupler 702 through a second coupler 714 and an opticalfiber 716. Interferometer 706 further may be coupled to a photodetectorassembly 718 of the DAS system through a third coupler 720 oppositesecond coupler 714. Second coupler 714 and third coupler 720 may be atraditional fused type fiber optic splitter, a PLC fiber optic splitter,or any other type of optical splitter known to those with ordinary skillin the art. Photodetector assembly 718 may include associated optics andsignal processing electronics (not shown). Photodetector assembly 718may be a semiconductor electronic device that uses the photoelectriceffect to convert light to electricity. Photodetector assembly 718 maybe an avalanche photodiode or a pin photodiode but is not intended to belimited to such.

When operating FOS system 126, pulse generator 700 may generate a firstoptical pulse 722 which is transmitted through optical fiber 704 tofirst coupler 702. First coupler 702 may direct first optical pulse 722through a sensing fiber 724. It should be noted that sensing fiber 724may be disposed in umbilical line 130 and is at least a part of downholesensing fiber 132 (e.g., referring to FIGS. 1A and 1B). As illustrated,sensing fiber 724 may be coupled to first coupler 702. As first opticalpulse 722 travels through sensing fiber 724, imperfections in sensingfiber 724 may cause a portion of the light to be backscattered alongfiber optical cable 724 due to Rayleigh scattering. In otherembodiments, the sensing fiber 724 may be enhanced (or engineered) toyield a higher-than-Rayleigh backscatter coefficient. Scattered lightaccording to Rayleigh scattering is returned from every point alongsensing fiber 724 along the length of sensing fiber 724 and is shown asbackscattered light 726 in FIG. 7 . This backscatter effect may bereferred to as Rayleigh backscatter. Density fluctuations in sensingfiber 724 may give rise to energy loss due to the scattered light,α_(scat), with the following coefficient:

$\begin{matrix}{\alpha_{scat} = {\frac{8\pi^{3}}{3\lambda^{4}}n^{8}p^{2}{kT}_{f}\beta}} & (1)\end{matrix}$

where n is the refraction index, p is the photoelastic coefficient ofsensing fiber 724, k is the Boltzmann constant, and β is the isothermalcompressibility. T_(f) is a fictive temperature, representing thetemperature at which the density fluctuations are “frozen” in thematerial. Fiber optical cable 724 may be terminated with a lowreflection device (not shown). In examples, the low reflection device(not shown) may be a fiber coiled and tightly bent to violate Snell'slaw of total internal reflection such that all the remaining energy issent out of sensing fiber 724.

Backscattered light 726 may travel back through sensing fiber 724, untilit reaches second coupler 714. First coupler 702 may be coupled tosecond coupler 714 on one side by optical fiber 716 such thatbackscattered light 726 may pass from first coupler 702 to secondcoupler 714 through optical fiber 716. Second coupler 714 may splitbackscattered light 726 based on the number of interferometer arms sothat one portion of any backscattered light 726 passing throughinterferometer 706 travels through top interferometer arm 708 andanother portion travels through bottom interferometer arm 710.Therefore, second coupler 714 may split the backscattered light fromoptical fiber 716 into a first backscattered pulse and a secondbackscattered pulse. The first backscattered pulse may be sent into topinterferometer arm 708. The second backscattered pulse may be sent intobottom interferometer arm 710. These two portions may be re-combined atthird coupler 720, after they have exited interferometer 706, to form aninterferometric signal.

Interferometer 706 may facilitate the generation of the interferometricsignal through the relative phase shift variations between the lightpulses in top interferometer arm 708 and bottom interferometer arm 710.Specifically, gauge 712 may cause the length of bottom interferometerarm 710 to be longer than the length of top interferometer arm 708. Withdifferent lengths between the two arms of interferometer 706, theinterferometric signal may include backscattered light from twopositions along sensing fiber 724 such that a phase shift ofbackscattered light between the two different points along sensing fiber724 may be identified in the interferometric signal. The distancebetween those points L may be half the length of the gauge 712 in thecase of a Mach-Zehnder configuration, or equal to the gauge length in aMichelson interferometer configuration.

While FOS system 126 is running, the interferometric signal willtypically vary over time. The variations in the interferometric signalmay identify strains in sensing fiber 724 that may be caused, forexample, by seismic energy. By using the time of flight for firstoptical pulse 722, the location of the strain along sensing fiber 724and the time at which it occurred may be determined. If sensing fiber724 is positioned within a wellbore, the locations of the strains insensing 724 may be correlated with depths in the formation in order toassociate the seismic energy with locations in the formation andwellbore.

To facilitate the identification of strains in sensing fiber 724, theinterferometric signal may reach photodetector assembly 718, where itmay be converted to an electrical signal. The photodetector assembly mayprovide an electric signal proportional to the square of the sum of thetwo electric fields from the two arms of the interferometer. This signalis proportional to:

P(t)=P1+P2+2*√{square root over ((P1P2)cos(ϕ1−ϕ2))}  (2)

where P_(n) is the power incident to the photodetector from a particulararm (1 or 2) and ϕ_(n) is the phase of the light from the particular armof the interferometer. Photodetector assembly 718 may transmit theelectrical signal to information handling system 146, which may processthe electrical signal to identify strains within sensing fiber 724and/or convey the data to a display and/or store it in computer-readablemedia. Photodetector assembly 718 and information handling system 146may be communicatively and/or mechanically coupled. Information handlingsystem 146 may also be communicatively or mechanically coupled to pulsegenerator 700.

Modifications, additions, or omissions may be made to FIG. 7 withoutdeparting from the scope of the present disclosure. For example, FIG. 7shows a particular configuration of components of a DAS system, which isa FOS system 126, operating via optical time-domain reflectometry(OTDR). However, any suitable configurations of components may be used,including such that the DAS system may be operated via opticalfrequency-domain interferometry (OFDR). As another example, pulsegenerator 700 may generate a multitude of coherent light pulses, opticalpulse 722, operating at distinct frequencies that are launched into thesensing fiber 724 either simultaneously or in a staggered fashion. Forexample, the photo detector assembly is expanded to feature a dedicatedphotodetector assembly for each light pulse frequency. In examples, acompensating interferometer may be placed in the launch path (i.e.,prior to traveling down sensing fiber 724) of the interrogating pulse togenerate a pair of pulses that travel down sensing fiber 724. Inexamples, interferometer 706 may not be necessary to interfere thebackscattered light from pulses prior to being sent to photo detectorassembly. In one branch of the compensation interferometer in the launchpath of the interrogating pulse, an extra length of fiber not present inthe other branch (a gauge length similar to gauge 712 of FIG. 7 ) may beused to delay one of the pulses. To accommodate phase detection ofbackscattered light using FOS system 126, one of the two branches mayinclude an optical frequency shifter (for example, an acousto-opticmodulator) to shift the optical frequency of one of the pulses, whilethe other may include a gauge. This may allow using a singlephotodetector receiving the backscatter light to determine the relativephase of the backscatter light between two locations by examining theheterodyne beat signal received from the mixing of the light fromdifferent optical frequencies of the two interrogation pulses.

In examples, the DAS system, which is a FOS system 126, may generateinterferometric signals for analysis by the information handling system146 without the use of a physical interferometer. For instance, the DASsystem may direct backscattered light to photodetector assembly 718without first passing it through any interferometer, such asinterferometer 706 of FIG. 7 . Alternatively, the backscattered lightfrom the interrogation pulse may be mixed with the light from the laseroriginally providing the interrogation pulse. Thus, the light from thelaser, the interrogation pulse, and the backscattered signal may all becollected by photodetector assembly 718 and then analyzed by informationhandling system 146. The light from each of these sources may be at thesame optical frequency in a homodyne phase demodulation system or may bedifferent optical frequencies in a heterodyne phase demodulator. Thismethod of mixing the backscattered light with a local oscillator allowsmeasuring the phase of the backscattered light along the fiber relativeto a reference light source.

FIG. 8 illustrates an example of s DAS system, which is a FOS system126, which may be utilized to overcome challenges presented by a subseaenvironment. The FOS system 126 may include interrogator unit 128,umbilical line 130, and downhole sensing fiber 132. As illustrated,interrogator unit 128 may include pulse generator 700 and photodetectorassembly 718, both of which may be communicatively coupled toinformation handling system 146. Additionally, interferometers 706 maybe placed within interrogator unit 128 and operate and/or function asdescribed above. FIG. 8 illustrates an example of FOS system 126 inwhich lead lines 800 may be used. As illustrated, an optical fiber 704may attach pulse generator 700 to an output 802, which may be a fiberoptic connector. Umbilical line 130 may attach to output 802 with afirst fiber optic cable 804. First fiber optic cable 804 may traversethe length of umbilical line 130 to a remote circulator 806. Remotecirculator 806 may connect first fiber optic cable 804 to second fiberoptic cable 808. In examples, remote circulator 806 functions to steerlight unidirectionally between one or more input and outputs of remotecirculator 806. Without limitation, remote circulators 806 are passivethree-port devices wherein light from a first port is split internallyinto two independent polarization states and wherein these twopolarization states are made to propagate two different paths insideremote circulator 806. These two independent paths allow one or bothindependent light beams to be rotated in polarization state via theFaraday effect in optical media. Polarization rotation of the lightpropagating through free space optical elements within the circulatorthus allows the total optical power of the two independent beams touniquely emerge together with the same phase relationship from a secondport of remote circulator 806.

Conversely, if any light enters the second port of remote circulator 806in the reverse direction, the internal free space optical elementswithin remote circulator 806 may operate identically on the reversedirection light to split it into two polarizations states. Afterappropriate rotation of polarization states, these reverse in directionpolarized light beams, are recombined, as in the forward propagationcase, and emerge uniquely from a third port of remote circulator 806with the same phase relationship and optical power as they had beforeentering remote circulator 806. Additionally, as discussed below, remotecirculator 806 may act as a gateway, which may only allow chosenwavelengths of light to pass through remote circulator 806 and pass todownhole sensing fiber 132. Second fiber optic cable 808 may attachumbilical line 130 to input 810. Input 810 may be a fiber opticconnector which may allow backscatter light to pass into interrogatorunit 128 to interferometer 706 Interferometer 706 may operate andfunction as described above and further pass back scatter light tophotodetector assembly 718.

FIG. 9 illustrates another example of FOS system 126, which is a DASsystem. As illustrated, interrogator unit 128 may include one or moreDAS interrogator units 900, each emitting coherent light pulses at adistinct optical wavelength, and a Raman Pump 902 connected to awavelength division multiplexer 904 (WDM) with fiber stretcher(s). Inexamples, any type of optical amplifier may be utilized in place ofRaman Pump 902. Without limitation, WDM 904 may include a multiplexerassembly that multiplexes the light received from the one or more DASinterrogator units 900 and a Raman Pump 902 onto a single optical fiberand a demultiplexer assembly that separates the multi-wavelengthbackscattered light into its individual frequency components andredirects each single wavelength backscattered light stream back to thecorresponding DAS interrogator unit 900. In an example, WDM 904 mayutilize an optical add-drop multiplexer to enable multiplexing the lightreceived from the one or more DAS interrogator units 900 and a RamanPump 902 and demultiplexing the multi-wavelength backscattered lightreceived from a single fiber. WDM 904 may also include circuitry tooptically amplify the multi-frequency light prior to launching it intothe single optical fiber and/or optical circuitry to optically amplifythe multi-frequency backscattered light returning from the singleoptical fiber, thereby compensating for optical losses introduced duringoptical (de-)multiplexing. Raman Pump 902 may be a co- and/orcounter-propagating optical pump based on stimulated Raman scattering,to feed energy from a pump signal to a main pulse from one or more DASinterrogator units 900 as the main pulse propagates down one or morefiber optic cables. This may conservatively yield a distributed 3 dBimprovement in SNR at 25 km tie-back distances, but as much as adistributed 6 dB gain at 50 km. Moreover, Raman Pump 902 may consist ofcascading Raman amplifiers assembled to yield a consistent distributedgain profile along the transmission fiber(s). As illustrated, Raman Pump902 is located in interrogator unit 128 for co-propagation. In anotherexample, Raman Pump 902 may be located topside after one or more remotecirculators 806 either in line with first fiber optic cable 804(co-propagation mode) and/or in line with second fiber optic cable 808(counter-propagation). In another example, Raman Pump 902 is marinizedand located after distal circulator 908 configured either forco-propagation or counter-propagation. In still another example, thelight emitted by the Raman Pump 902 is remotely reflected by using awavelength-selective filter beyond a remote circulator in order toprovide amplification in the return path using a Raman Pump 902 in anyof the topside configurations outlined above. The wavelength-selectivefilter beyond the remote circulator may also be used to ensure the highoptical power of the Raman Pump 902 is reflected from low power opticalcomponents, such as the optical wet-mate connectors in the opticalfeedthrough system 144 (e.g., referring to FIGS. 6A and 6B).

Further illustrated in FIG. 9 , WDM 904 with fiber stretcher may attachproximal circulator 906 to umbilical line 130. Umbilical line 130 mayinclude one or more remote circulators 806, a first fiber optic cable804, and a second fiber optic cable 808. As illustrated, a first fiberoptic cable 804 and a second fiber optic cable 808 may be separate andindividual fiber optic cables that may be attached at each end to one ormore remote circulators 806. In examples, first fiber optic cable 804and second fiber optic cable 808 may be different lengths or the samelength and each may be an ultra-low loss transmission fiber that mayhave a higher power handling capability before non-literarily. This mayenable a higher gain, co-propagation Raman amplification frominterrogator unit 128.

Deploying first fiber optic cable 804 and as second fiber optic cable808 from floating vessel 102 (e.g., referring to FIGS. 1A and 1B) to asubsea environment to a distal-end passive optical circulatorarrangement, enables downhole sensing fiber 132, which is a sensingfiber, to be below a remote circulator 806 (e.g., well-only) that may beat the distal end of FOS system 126, which is a DAS system. This mayallow for higher (e.g., 2-3×) DAS pulse repetition rates, and allow forthe optical receivers to be adjusted such that their dynamic range isoptimized for downhole sensing fiber 132. Depending on the tie-backdistance between OFL and interrogator, this may yield 3 to 10 dBimprovement in SNR. Additionally, downhole sensing fiber 132 may be anenhanced backscatter sensing fiber that has higher-than-Rayleighscattering coefficient which may result in a ten to one hundred timesimprovement in backscatter, which may yield a 10 dB to 20 dB improvementin SNR. In examples, remote circulators 806 may further be categorizedas a proximal circulator 906 and a distal circulator 908. Proximalcirculator 906 is located closer to interrogator unit 128 and may belocated on floating vessel 102 or within umbilical line 130. Distalcirculator 908 may be further away from interrogator unit 128 thanproximal circulator 906 and may be located in umbilical line 130, in anoptical flying lead 142, in an optical feedthrough system 144, or withinwellbore 122 (e.g., referring to FIGS. 1A and 1B). As discussed above, aconfiguration illustrated in FIG. 8 may not utilize a proximalcirculator 906 with lead lines 800.

FIG. 10 illustrates another example of distal circulator 908, which mayinclude two remote circulators 806. As illustrated, each remotecirculator 806 may function and operate to avoid overlap, atinterrogator unit 128, of backscattered light from two different pulses.For example, during operations, light at a first wavelength may travelfrom interrogator unit 128 down first fiber optic cable 804 to a remotecirculator 806. As the light passes through remote circulator 806 thelight may encounter a Fiber Bragg Grating 1000. In examples, Fiber BraggGrating 1000 may be referred to as a filter mirror that may be awavelength specific high reflectivity filter mirror or filter reflectorthat may operate and function to recirculate unused light back throughthe optical circuit for “double-pass” co- and/or counter-propagatingRaman amplification of the DAS signal. In examples, Fiber Bragg Grating1000 may be referred to as an optically reflective element. In examples,this wavelength specific “Raman light” mirror may be a dichroic thinfilm interference filter, Fiber Bragg Grating 1000, or any othersuitable optical filter that passes only the 1550 nm forward propagatingDAS interrogation pulse light while simultaneously reflecting most ofthe residual Raman Pump light.

Without limitation, Fiber Bragg Grating 1000 may be set-up, fabricated,altered, and/or the like to allow only certain selected wavelengths oflight to pass. All other wavelengths may be reflected back to the secondremote circulator, which may send the reflected wavelengths of lightalong second fiber optic cable 808 back to interrogator unit 128. Thismay allow Fiber Bragg Grating 1000 to split DAS system 126 (e.g.,referring to FIG. 9 ) into two regions. A first region may be identifiedas the devices and components before Fiber Bragg Grating 1000 and thesecond region may be identified as downhole sensing fiber 132 and anyother devices after Fiber Bragg Grating 1000.

Splitting the DAS system, which is a FOS system 126, (e.g., referring toFIG. 9 ) into two separate regions may allow interrogator unit 128(e.g., referring to FIGS. 1A and 1B) to pump specifically for anidentified region. For example, the disclosed system of FIG. 9 mayinclude one or more Raman pumps 902, as described above, placed ininterrogator unit 128 or after proximal circulator 906 at the topsideeither in line with first fiber optic cable 804 or second fiber opticcable 808 that may emit a wavelength of light that may travel only to afirst region and be reflected by Fiber Bragg Grating 1000. A secondRaman pump may emit a wavelength of light that may travel to the secondregion by passing through Fiber Bragg Grating 1000. Additionally, boththe first Raman pump and second Raman pump may transmit at the sametime. Without limitation, there may be any number of Raman pumps and anynumber of Fiber Bragg Gratings 1000 which may be used to control whatwavelength of light travels through downhole sensing fiber 132. FIG. 10also illustrates Fiber Bragg Gratings 1000 operating in conjunction withany remote circulator 806, whether it is a distal circulator 908 or aproximal circulator 906. Additionally, as discussed below, Fiber BraggGratings 1000 may be attached at the distal end of downhole fiber 218and act as a mirror. Other alterations to DAS system 126 (e.g.,referring to FIG. 9 ) may be undertaken to improve the overallperformance of DAS system 126. For example, the lengths of first fiberoptic cable 804 and second fiber optic cable 808 may be selected toincrease pulse repetition rate (expressed in terms of the time intervalbetween pulses t_(rep)).

FIG. 11 illustrates an example of fiber optic cable 1100 in which noremote circulator 806 may be used. As illustrated, the entire fiberoptic cable 1100 is a sensor and the pulse interval must be greater thanthe time for the pulse of light to travel to the end of fiber opticcable 1100 and its backscatter to travel back to interrogator unit 128(e.g., referring to FIGS. 1A and 1B). This is so, since in the DASsystem, which is a FOS system 126, at no point in time, backscatter frommore than one location along sensing fiber (i.e., downhole sensing fiber132) may be received. Therefore, the pulse interval t_(rep) must begreater than twice the time light takes to travel “one-way”down thefiber. Let t_(s) be the “two-way” time for light to travel to the end offiber optic cable 1100 and back, which may be written as t_(rep)>t_(s).

FIG. 12 illustrates an example of fiber optic cable 1100 with a remotecirculator 806 using the configuration shown in FIG. 9 . When a remotecirculator 806 is used, only the light traveling in fiber optic cable600 that is allowed to go beyond remote circulator 806 and to downholesensing fiber 132 may be returned to interrogator unit 128 (e.g.,referring to FIGS. 1A and 1B), thus, the interval between pulses isdictated only by the length of the sensing portion, downhole sensingfiber 132 of fiber optic cable 1100. It should be noted that in terms ofpulse timing what matters is the two-way travel time of the light pulse“to” and “from” the sensing portion, downhole sensing fiber 132.Therefore, the first fiber optic cable 804 or second fiber optic cable808 “to” and “from” remote circulator 806 may be longer than the other,as discussed above.

FIG. 13 illustrates an example remote circulator arrangement 1300 whichmay allow, as described above, configurations that use more than oneremote circulator 806 close together at the remote location. Althoughremote circulator arrangement 1300 may have any number of remotecirculators 806, remote circulator arrangement 1300 may be illustratedas a single remote circulator 806.

FIG. 14 illustrates an example first fiber optic cable 804 and secondfiber optic cable 808 attached to a remote circulator 806 at each end.As discussed above, each remote circulator may be categorized as aproximal circulator 906 and a distal circulator 908. When using aproximal circulator 906 and a distal circulator 908, light from thefiber section before proximal circulator 906, and light from the fibersection below the remote circular 806 are detected, which is illustratedin FIGS. 15A-16 . There is a gap 1500 between them of “no light” thatdepends on the total length of fiber (summed) between proximalcirculator 906 and a distal circulator 908 (e.g., referring to FIG. 14).

Referring back to FIG. 14 , with t_(s1) the duration of the light fromfiber sensing section before proximal circulator 906, t_(sep) the “deadtime” separating the two sections (and due to the cumulative length offirst fiber optic cable 804 and second fiber optic cable 808 betweenproximal circulator 906 and a distal circulator 908), and t_(s2) theduration of the light from the sensing fiber, downhole sensing fiber132, beyond distal circulator 908, the constraints on fiber lengths andpulse intervals may be identified as:

i. t _(rep) <t _(sep)  (3)

ii. (2t _(rep))>(t _(s1) +t _(sep) t _(s2))  (4)

Criterion (i) ensures that “pulse n” light from downhole sensing fiber132 does not appear while “pulse n+1” light from fiber before proximalcirculator 906 is being received at interrogator unit 128 (e.g.,referring to FIGS. 1A and 1B). Criterion (ii) ensures that “pulse n”light from downhole sensing fiber 132 is fully received before “pulsen+2” light from fiber before proximal circulator 906 is being receivedat interrogator unit 128. It should be noted that the two criteria givenabove only define the minimum and maximum t_(rep) for scenarios wheretwo pulses are launched in the fiber before backscattered light belowthe remote circulator 806 is received. However, it should be appreciatedthat for those skilled in the art these criteria may be generalized tocases where n E {1,2,3, . . . } light pulses may be launched in thefiber before backscattered light below the remote circulator 806 isreceived.

The use of remote circulators 806 may allow for FOS system 126, a DASsystem, (e.g., referring to FIG. 8 ) to increase the DAS pulserepetition rate, or sampling frequency. FIG. 17 illustrates workflow1700 for optimizing sampling frequency when using a remote circulator806 in FOS system 126. One skilled in the art will appreciate thesubtlety that optimizing the sampling frequency doesn't imply maximizingthe sampling frequency. Workflow 1700 may begin with block 1702, whichdetermines the overall fiber length in both directions. For example, incase of a 17 km of first fiber optic cable 804 and 17 km of second fiberoptic cable 808 before distal circulator 908 and 8 km of sensing fiber,downhole sensing fiber 132, after distal circulator 908, the overallfiber optic cable length in both directions would be 50 km. Assuming atravel time of the light of 5 ns/m, the following equation may be usedto calculate a first DAS sampling frequency f_(s)

$\begin{matrix}{f_{s} = {\frac{1}{t_{s}} = \frac{1}{5 \cdot 10^{- 9} \cdot z}}} & (5)\end{matrix}$

where t_(s) is the DAS sampling interval and z is the overall two-wayfiber length. Thus, for an overall two-way fiber length of 50 km thefirst DAS sampling rate f_(s) is 4 kHz. In block 1704 regions of thefiber optic cable are identified for which backscatter is received. Forexample, this is done by calculating the average optical backscatteredenergy for each sampling location followed by a simple thresholdingscheme. The result of this step is shown in FIG. 15A where boundaries1502 identify two sensing regions 1504. As illustrated in FIG. 15A-15C,optical energy is given as:

l ² +Q ²  (6)

where I and Q correspond to the in-phase (I) and quadrature (Q)components of the backscattered light. In block 1706, the samplingfrequency of FOS system 126, a DAS system, is optimized. To optimize thesampling frequency a minimum time interval is found that is between theemission of light pulses such that at no point in time backscatteredlight arrives back at interrogator unit 128 (e.g., referring to FIG. 1 )that corresponds to more than one spatial location along a sensingportion of the fiber-optic line. Mathematically, this may be defined asfollows. Let S be the set of all spatial sample locations x along thefiber for which backscattered light is received. The desired light pulseemission interval t_(s) is the smallest one for which the cardinality ofthe two sets S and {mod(x, t_(s)): x ∈S} is still identical, which isexpressed as:

$\begin{matrix}\begin{matrix}{\min\limits_{t_{s}}\left( t_{s} \right)} & {s.t.} & {{❘S❘} = {❘\left\{ {{{{mod}\left( {x,t_{s}} \right)}:x} \in S} \right\} ❘}}\end{matrix} & (7)\end{matrix}$

where |.| is the cardinality operator, measuring the number of elementsin a set. FIG. 16 shows the result of optimizing the sampling frequencyfrom FIGS. 10A-10C with workflow 1700. Here, the DAS sampling frequencymay increase from 4 kHz to 12.5 kHz without causing any overlap inbackscattered locations, effectively increasing the signal to noiseratio of the underlying acoustic data by more than 5 dB due to theincrease in sampling frequency.

Variants of FOS system 126, which may be DAS based, may also benefitfrom workflow 1700. For example, FIG. 18 illustrates FOS system 126 inwhich proximal circulator 906 is placed within interrogator unit 128.This system set up of FOS system 126 may allow for system flexibility onhow to implement during measurement operations and the efficientplacement of Raman Pump 1900. As illustrated in FIGS. 19 and 20 , firstfiber optic cable 804 and second fiber optic cable 808 may connectinterrogator unit 128 to umbilical line 130, which is described ingreater detail above in FIG. 8 .

FIG. 19 illustrates another example of DAS system 126 in which RamanPump 1900 is operated in co-propagation mode and is attached to firstfiber optic cable 804 after proximal circulator 906. For example, if thefirst sensing region before proximal circulator 906 should not beaffected by Raman amplification. Moreover, Raman Pump 1900, may also beattached to second fiber optic cable 808 which may allow the Raman Pump1900 to be operated in counter-propagation mode. In examples, the RamanPump may also be attached to fiber 1902 between WDM 904 and proximalcirculator 906 in interrogator unit 128.

FIG. 20 illustrates another example of FOS system 126 in which anoptical amplifier assembly 2000 (i.e., an Erbium doped fiber amplifier(EDFA)+Fabry-Perot filter) may be attached to proximal circulator 906,which may also be identified as a proximal locally pumped opticalamplifier. In examples, a distal optical amplifier assembly 2002 mayalso be attached at distal circulator 908 on first fiber optical cable804 or second fiber optical cable 808 as an inline or “mid-span”amplifier. In examples, optical amplifier assembly 2002 located in-linewith fiber optical cable 804 and above distal circulator 908 may be usedto boost the light pulse before it is launched into the downhole sensingfiber 132. Referring to FIGS. 10B and 10C, the effect of using anoptical amplifier assembly 2000 in-line with a second fiber optic cable808 prior to proximal circulator 906 and/or using an distal opticalamplifier assembly 2002 located in line with second fiber optical cable808 above distal circulator 908 may allow for selectively amplifying thebackscattered light originating from downhole sensing fiber 132 whichtends to suffer from much stronger attenuation as it travels back alongdownhole sensing fiber 132 and second fiber optical cable 808 thanbackscattered light originating from shallower sections of fiber opticcable that may also perform sensing functions. FIG. 10B illustratesmeasurements where proximal circulator 906 is active (optical amplifierassembly 2000 in-line with a second fiber optic cable 808 prior toproximal circulator 906 and/or distal optical amplifier assembly 2002located in line with second fiber optical cable 808 above distalcirculator 908 is used). FIG. 10C illustrates measurements whereproximal circulator 906 is passive (no optical amplification is usedin-line with second fiber optic cable 808). In FIGS. 10B and 10C,boundaries 1502 identify two sensing regions 1504. Additionally, inFIGS. 10B and 10C the DAS sampling frequency is set to 12.5 kHz usingworkflow 1700. Further illustrated Fiber Bragg Grating 1000 may also bedisposed on first fiber optical cable 804 between distal opticalamplifier assembly 2002 and distal circulator 908.

During operation, data quality from a FOS system 126, such as a DASsystem, (e.g., referring to FIG. 7 ) may be governed by signal qualityand sampling rate. Signal quality is predominantly constrained by thepower of backscattered light and sampling rate is constrained by sensingfiber length. For example, the less backscattered light that is receivedfrom a sensing fiber, which may be downhole sensing fiber 132 ordisposed on downhole sensing fiber 132 (e.g., referring to FIGS. 1A and1B), the more inferior the quality of the measurement taken by FOSsystem 126.

FIG. 21 illustrates an example of FOS system 126 (e.g., referring toFIGS. 1A and 1B) in which multiple downhole sensing fibers 132 areutilized. As discussed above, in reference to FIGS. 6B, legacy opticalfeedthrough system technology connects a single fiber from an opticalflying lead 142 to a single downhole sensing fiber 132. FIG. 21illustrates an example FOS system 126 that may utilize a singleumbilical line 130 to service multiple downhole sensing fibers 132through a single connection. Generally, at surface on vessel 102 (e.g.,referring to FIGS. 1A and 1B) FOS system 126 may originate withinterrogator unit 128. Interrogator unit 128 may include one or moreinterrogator units 900, including but not limited to DAS, DTS, DSS,DBFS, and/or FBG interrogators, each emitting light pulses at a distinctoptical wavelength, connected to at least one wavelength divisionmultiplexer 904 (WDM) disposed in WDM compartment 2100. It should benoted that WDM 904 may also be referred to as a “proximal WDM.” Thus,WDM compartment 2100 may be separate and apart from interrogator unit128. However, in example, WDM compartment 2100 may be merged intointerrogator unit 128. In other examples, WDM 904 may not be disposed ina WDM compartment 2100 but may be integrated into interrogator unit 128,umbilical line 130, optical flying lead 142, optical feedthrough system144, and/or one or more downhole sensing fibers 132. WDM 904 may containwith fiber stretchers. Without limitation, WDM 904 may include amultiplexer assembly that multiplexes the light received from theplurality of interrogator units 900 onto a single optical fiber and ademultiplexer assembly that separates the multi-wavelength backscatteredlight into its individual frequency components and redirects each singlewavelength backscattered light stream back to the correspondinginterrogator unit 900. In an example, WDM 904 may utilize an opticaladd-drop multiplexer to enable multiplexing the light received from theone or more interrogator units 900 and demultiplexing themulti-wavelength backscattered light received from a single fiber. WDM904 may also include circuitry to optically amplify the multi-frequencylight prior to launching it into the single optical fiber and/or opticalcircuitry to optically amplify the multi-frequency backscattered lightreturning from the single optical fiber, thereby compensating foroptical losses introduced during optical (de-)multiplexing.

As illustrated, interrogator unit 128 may connect to WDM compartment2100, which may connect to umbilical line 130. In examples, lightoriginating from WDM 904 may interact with a proximal circulator 906. Inexamples, proximal circulator 906 may be disposed in WDM compartment2100. In other examples, proximal circulator 906 may be disposed withinumbilical line 130 or interrogator unit 128 (as seen in FIGS. 18-20 ).Moving through proximal circulator 906, light may traverse through firstfiber optic cable 804, which may also be identified as a “down-goingtransmission fiber.” The light may then pass from umbilical line 130 tooptical flying lead 142, as discussed above. Within flying optical lead142 containing an integrated compartment 502 (e.g., referring to FIG. 5), one or more WDM 905 and one or more distal circulators 908 may bedisposed. It should be noted that WDM 905 may be referred to as “distalWDM” or “distal down-going WDM.” Additionally, as noted above,integrated compartment 502 may be disposed in umbilical line 130,optical feedthrough system 144, or between optical feedthrough system144 and downhole sensing fibers 132. WDM 905 may operate and function tosplit light from first fiber optic cable 804 into one or more fiberoptic cables 2102 within integrated compartment 502. Each fiber opticcable 2102 may connect to a distal circulator 908 which are alsodisposed in integrated compartment 502. Each fiber optical cable 2102may connect to a downhole sensing fiber 132 through optical feedthroughsystem 144 as described above in FIGS. 6A and 6B.

Each downhole sensing fiber 132 may be include one or more fiber opticsensors 2104. Additionally, some downhole sensing fiber 132 may notinclude any fiber optic sensors 2104 and may be used for distributedacoustic and/or temperature measurements of the optical fiber. Fiberoptic sensors 2104 may include, but are not limited to fiber opticpressure, temperature, chemical, and/or voltage sensors. Lighttraversing downhole sensing fiber 132 may generate backscatter, whichtraverses thorough optical feedthrough system 144 and back to opticalflying lead 142. At some point, the backscatter light enters integratedcompartment 502. In integrated compartment 502 the backscattered lightmay interact with distal circulator 908 on each fiber optic cable 2102.Distal circulator 908 may route backscattered light through secondaryfiber optic cable 2106, which leads to another WDM 905. It should benoted that WDM 905 may also be referred to as “distal WDM” or “distalup-going WDM.” WDM 905 may then operate and function as described aboveto combine light form each secondary fiber optic cables 2106 into secondfiber optic cable 808, which may also be identified as “upgoingtransmission fiber.” In other examples, WDM 905 may not be disposed inintegrated compartment 502 but may be integrated into interrogator unit128, umbilical line 130, optical flying lead 142, optical feedthroughsystem 144, and/or one or more downhole sensing fibers 132. Similar toFIG. 20 , in examples, an optical amplifier assembly 2000 may be placedin-line with a second fiber optic cable 808 prior to proximal circulator906, within umbilical line 130. Backscatter light traversing throughsecond fiber optic cable 808 may then interact with proximal circulator906, which may direct backscatter light to interrogator unit 128 to bemeasured and/or recorded as described above. One skilled in the art willappreciate that the distal assembly of WDM 905 and circulators 908 maybe integrated in an optical flying lead 142, optical feedthrough system144, umbilical line 130, or maybe integrated elsewhere in the subseaoptical distribution system as matter of convenience as they may becontained in integrated compartment 502, which may be disposed at anyspot in FOS system 126.

FIG. 22 illustrates and example of FOS system 126, as illustrated inFIG. 21 , with a Raman Pump 902 which is connected to a WDM 904, whichis disposed in WDM compartment 2100. Raman Pump 902 may be aco-propagating optical pump based on stimulated Raman scattering, tofeed energy from a pump signal to a main pulse from one or more FOSinterrogator units 900 as the main pulse propagates down one or morefiber optic cables. As illustrated, Raman Pump 902 is located ininterrogator unit 128 for co-propagation. In another example, Raman Pump902 may be located topside either in line with first fiber optic cable804 (co-propagation mode) and/or in line with second fiber optic cable808 (counter-propagation) in WDM compartment 2100. In another example,Raman Pump 902 is marinized and located after distal circulator 908configured either for co-propagation or counter-propagation. In stillanother example, the light emitted by the Raman Pump 902 is remotelyreflected by using a wavelength-selective filter beyond distalcirculator 908 in order to provide amplification in the return pathusing a Raman Pump 902 in any of the topside configurations outlinedabove. The wavelength-selective filter beyond distal circulator 908 mayalso be used to ensure the high optical power of Raman Pump 902 isreflected from low power optical components, such as the opticalwet-mate connectors in optical feedthrough system 144.

With continued reference to FIG. 22 , a WDM 905 in integratedcompartment 502 may split light coming from first fiber optic cable 804.Integrated compartment 502 may be disposed at any location in FOS system126 as described above. When splitting light in WDM 905 Raman light fromRaman Pump 902 may enter a fiber optic cable 2102 with a dedicateddistal circulator 908. Raman Pump light may traverse through fiber opticcable 2102 and through distal circulator 908 to a Fiber Bragg Grating1000, which may be referred to as a filter mirror that may be awavelength specific high reflectivity filter mirror or filter reflectorthat may operate and function to recirculate unused light back throughthe optical circuit for “double-pass” co/counter propagation Ramanamplification of the FOS signals. In examples, this wavelength specific“Raman light” mirror may be a dichroic thin film interference filter,Fiber Bragg Grating 1000, or any other suitable optical filter thatpasses only the forward propagating FOS interrogation pulse light whilesimultaneously reflecting most of the residual Raman Pump light. Thereflected Raman Pump light may traverse back through distal circulator908 and through secondary fiber optic cables 2104 to a second WDM 905,which may recombine backscatter light and the Raman Pump light. This mayallow for the backscatter light to traverse back up umbilical line 130to interrogator unit 128.

FIG. 23 illustrates another example of FOS system 126 where proximalcirculator 906 and distal circulator 908 may be disposed in umbilicalline 130, similar to examples in FIGS. 9 and 20 . In such example, WDM905 may be a single device in integrated compartment 502. From WDM 905,individual fiber optic cables 2102 may mate with downhole sensing fibers132 through optical feedthrough system 144 as discussed above.Backscatter light may flow from downhole sensing fibers 132 back to WDM905 in integrated compartment 502 to be recombined, as discussed above.From WDM 905 in integrated compartment 502, backscatter light mayinteract with distal circulator 908 disposed in umbilical line 130,which may move backscatter light into second fiber optic cable 808 andoptical amplifier assembly 2000, which is discussed in detail above.Backscatter light may then interact with proximal circulator 906,disposed in WDM compartment 2100, where it is directed back tointerrogator unit 128 to be measured and recorded.

FIG. 24 illustrates another example of FOS system 126 with a Raman Pump902. Raman Pump 902 may function and/or operate as discussed above forFIGS. 9 and 22 . As illustrates, in integrated compartment 502, WDM 905may split out Raman light from Raman Pump 902 and may enter a fiberoptic cable 2102 with a dedicated Fiber Bragg Grating 1000, which may bereferred to as a filter mirror that may be a wavelength specific highreflectivity filter mirror or filter reflector that may operate andfunction to recirculate unused light back through the optical circuitfor “double-pass” co/counter propagation Raman amplification of the FOSsignals. In examples, this wavelength specific “Raman light” mirror maybe a dichroic thin film interference filter, Fiber Bragg Grating 1000,or any other suitable optical filter that passes only the forwardpropagating FOS interrogation pulse light while simultaneouslyreflecting most of the residual Raman Pump light. The reflected RamanPump light may traverse back through WDM 905, which may recombinebackscatter light and the Raman Pump light in integrated compartment502. This may allow for the backscatter light to traverse back upumbilical line 130, through WDM 904 in WDM compartment 2100 and tointerrogator unit 128.

FIGS. 21 to 24 have shown dual transmission fibers leading to the distalWDM and circulator assembly, whether the distal WDM and circulatorassembly is integrated in an optical flying lead, integrated in anoptical feedthrough system, or integrated elsewhere in the subseaoptical distribution system as matter of convenience in an integratedcompartment. This dual transmission fiber configuration enablesoptimization of the FOS pulse repetition rates for sensing the downholesensing fiber, for data quality and fidelity advantages previouslydescribed. One skilled in the art will appreciate that a simplerembodiment may only employ a distal WDM and no circulators. This wouldforgo optimization of the FOS pulse repetition rates for sensing thedownhole sensing fiber but would still enable multiple interrogators tosense multiple downhole sensing fibers with a single transmission fiberproviding optical continuity between the interrogators and downholesensing fibers. The distal WDM may be integrated in an optical flyinglead, integrated in an optical feedthrough system, or integratedelsewhere in the subsea optical distribution system as matter ofconvenience in an integrated compartment.

FIG. 25 illustrates an example of an onshore well system 2500, whichillustrates downhole sensing fibers 132 permanently installed in thecompletion of an onshore well. Given the use of a dry Christmas tree (ordry-tree), the optical feedthrough system of the subsea tree may besimplified with an appropriate wellhead exit. As illustratedinterrogator unit 128 is attached to information handling system 146.Further discussed below, lead lines may connect umbilical line 130 (orsurface cables) to interrogator unit 128. Umbilical line 130 may be asurface cable, or a trenched cable, which can include a first fiberoptic cable 804 and a second fiber optic cable 808 which may beindividual lead lines. Without limitation, first fiber optic cable 804and a second fiber optic cable 808 may attach to completion system 2502as umbilical line 130. Umbilical line 130 may traverse through wellbore122 attached to completion system 2502. Further illustrated in FIG. 25 ,umbilical line 130 may connect to integrated compartment 502, which mayconnect umbilical line 130 to one or more downhole sensing fiber 132.This may be performed, function, and/or operate as described above.

Systems and methods described functionally provides an all-opticaldownhole sensing solution for subsea wells, enabling the simultaneousmeasurements of temperature, pressure, acoustics, and/or strain indownhole sensing fibers. The system and methods described are inherentlycompliant with the Intelligent Well Interface Standardization (IWIS) andSEAFOM recommended practices. Systems and methods described functionallyprovides an all-optical downhole sensing solution for subsea wells. Inpractice, the systems and methods may minimize the number oftransmission fibers providing optical continuity from topside to opticalflying lead, thus saving significant complexity and costs in subseaoptical infrastructure and installation thereof. Additionally, systemsand methods described above can maximize the number of downhole sensingfibers that can be configured for any combination of fiber optic sensingapplications. In particular, the systems and methods can enablesimultaneous DAS, DSS, DTS, and FBG sensing of subsea completions.

By retaining all electro-optical systems, such as interrogator systems,at the topside, the systems and methods described can eliminate the needfor electric downhole sensing systems and their related subsea controlsand power distribution systems. For example, to operate an array ofelectric pressure and temperature gauges across the reservoir using aninductive coupler for power and telemetry between the upper and lowercompletions introduces significant cost and complexity to the subseapower distribution system. Moreover, interfaces between the electricdownhole sensors and the subsea tree control module are eliminated;further simplifying subsea control systems.

The systems and methods for a fiber optic sensing system discussedabove, implemented within a subsea environment may include any of thevarious features of the systems and methods disclosed herein, includingone or more of the following statements. Moreover, the systems andmethods for a fiber optic sensing system discussed above implementedwithin an onshore environment may include any of the various features ofthe systems and methods disclosed herein, including one or more of thefollowing statements.

Statement 1: A fiber optic sensing (FOS) system may include one or moreinterrogator units, a proximal wavelength division multiplexer (WDM)optically connectable to the one or more interrogator units, an upgoingtransmission fiber, and a down-going transmission fiber, a distal WDMoptically connectable to the upgoing transmission fiber and thedown-going transmission fiber, and one or more downhole sensing fibersoptically connectable to the distal WDM.

Statement 2: The FOS system of statement 1, wherein the distal WDM isdisposed in an optical flying lead, an optical feedthrough system, or anoptical distribution unit.

Statement 3: The FOS system of statements 1 or 2, wherein the one ormore interrogator units are configured for distributed fiber opticsensing or discrete fiber optic sensing of an acoustic measurement, avibration measurement, a strain measurement, a temperature measurement,a pressure measurement, a chemical measurement, or a voltage measurementin the one or more downhole sensing fibers.

Statement 4. The FOS system of statement 3, further comprising anoptical amplifier optically connectable to the one or more interrogatorunits.

Statement 5. The FOS system of statements 1-3, further comprises atleast one optical amplifier located between the proximal WDM and thedistal WDM.

Statement 6. The FOS system of statements 1-3 or 5, wherein the one ormore downhole sensing fibers are different lengths.

Statement 7. The FOS system of statements 1-3, 5, or 6, furthercomprising one or more distal circulators.

Statement 8. The FOS system of statement 7, wherein a distal circulatoris optically connectable to the distal WDM, the one or more downholesensing fibers, and a second distal WDM.

Statement 9. The FOS system of statement 8, wherein the second distalWDM is optically connectable to the upgoing fiber and the down goingfiber.

Statement 10. The FOS system of statements 1-3 or 5-7, wherein aproximal circulator is optically connectable to the proximal WDM and theupgoing fiber and the down going fiber.

Statement 11. The FOS system of statements 1-3, 5-7, or 10, furthercomprising a distal circulator that is optically connectable to thedistal WDM, an optically reflective element, and a second distal WDM.

Statement 12. A method for fiber optic sensing (FOS) may includetransmitting one or more light pulses from an interrogator unit that isoptically connected to a proximal wavelength division multiplexer (WDM),multiplexing the one or more light pulses from the interrogator unitwith the proximal WDM into an upgoing transmission fiber and adown-going transmission fiber, receiving the one or more light pulseswith a distal WDM that is optically connected to the upgoingtransmission fiber and the down-going transmission fiber, multiplexingthe one or more light pulses from the upgoing transmission fiber and thedown-going transmission fiber into one or more downhole sensing fibers,and receiving backscatter light from at least one of the one or moredownhole sensing fibers.

Statement 13. The method of statement 12, further comprising generatingone or more light pulses with at least one wavelength or within abandwidth of wavelengths with an interrogator disposed in theinterrogator unit.

Statement 14. The method of statement 13, further comprising combiningthe one or more light pulses from one or more interrogators with theproximal WDM.

Statement 15. The method of statement 13, wherein each interrogator isconfigured to launch light pulses at different pulse repetition rates.

Statement 16. The method of statements 12 or 13, wherein a distalcirculator is optically connected to the distal WDM, the one or moredownhole sensing fibers, and a second distal WDM.

Statement 17. The method of statement 16, wherein the second distal WDMis optically connected to the upgoing transmission fiber and thedown-going transmission fiber.

Statement 18. The method of statement 17, wherein a proximal circulatoris optically connected to the proximal WDM, the upgoing transmissionfiber, and the down-going transmission fiber.

Statement 19. The method of statements 12, 13, or 16, further includingoptimizing a sampling frequency by identifying a length of at least oneof the one or more downhole sensing fibers optically connected to thedistal WDM, identifying one or more sensing regions on the at least oneof the one or more downhole sensing fibers, and identifying a minimumtime interval that is between an emission of the one or more lightpulses such that at no point in time the backscattered light arrivesback at the interrogator unit during an emission of the one or morelight pulses.

Statement 20. The method of statements 12, 13, 16, or 19, furthercomprising performing a distributed fiber optic sensing measurement or adiscrete fiber optic sensing measurement in one or more sensing regionsof the one or more downhole sensing fibers.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations may be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. The precedingdescription provides various examples of the systems and methods of usedisclosed herein which may contain different method steps andalternative combinations of components. It should be understood that,although individual examples may be discussed herein, the presentdisclosure covers all combinations of the disclosed examples, including,without limitation, the different component combinations, method stepcombinations, and properties of the system. It should be understood thatthe compositions and methods are described in terms of “comprising,”“containing,” or “including” various components or steps, thecompositions and methods can also “consist essentially of” or “consistof” the various components and steps. Moreover, the indefinite articles“a” or “an,” as used in the claims, are defined herein to mean one ormore than one of the element that it introduces.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present examples are well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular examples disclosed above are illustrative only and may bemodified and practiced in different but equivalent manners apparent tothose skilled in the art having the benefit of the teachings herein.Although individual examples are discussed, the disclosure covers allcombinations of all of the examples. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. Also, the terms in the claimshave their plain, ordinary meaning unless otherwise explicitly andclearly defined by the patentee. It is therefore evident that theparticular illustrative examples disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of those examples. If there is any conflict in the usages of aword or term in this specification and one or more patent(s) or otherdocuments that may be incorporated herein by reference, the definitionsthat are consistent with this specification should be adopted.

What is claimed is:
 1. A fiber optic sensing (FOS) system comprising:one or more interrogator units; a proximal wavelength divisionmultiplexer (WDM) optically connectable to the one or more interrogatorunits, an upgoing transmission fiber, and a down-going transmissionfiber; a distal WDM optically connectable to the upgoing transmissionfiber and the down-going transmission fiber; and one or more downholesensing fibers optically connectable to the distal WDM.
 2. The FOSsystem of claim 1, wherein the distal WDM is disposed in an opticalflying lead, an optical feedthrough system, or an optical distributionunit.
 3. The FOS system of claim 1, wherein the one or more interrogatorunits are configured for distributed fiber optic sensing or discretefiber optic sensing of an acoustic measurement, a vibration measurement,a strain measurement, a temperature measurement, a pressure measurement,a chemical measurement, or a voltage measurement in the one or moredownhole sensing fibers.
 4. The FOS system of claim 3, furthercomprising an optical amplifier optically connectable to the one or moreinterrogator units.
 5. The FOS system of claim 1, further comprises atleast one optical amplifier located between the proximal WDM and thedistal WDM.
 6. The FOS system of claim 1, wherein the one or moredownhole sensing fibers are different lengths.
 7. The FOS system ofclaim 1, further comprising one or more distal circulators.
 8. The FOSsystem of claim 7, wherein a distal circulator is optically connectableto the distal WDM, the one or more downhole sensing fibers, and a seconddistal WDM.
 9. The FOS system of claim 8, wherein the second distal WDMis optically connectable to the upgoing fiber and the down going fiber.10. The FOS system of claim 1, wherein a proximal circulator isoptically connectable to the proximal WDM and the upgoing fiber and thedown going fiber.
 11. The FOS system of claim 1, further comprising adistal circulator that is optically connectable to the distal WDM, anoptically reflective element, and a second distal WDM.
 12. A method forfiber optic sensing (FOS) comprising: transmitting one or more lightpulses from an interrogator unit that is optically connected to aproximal wavelength division multiplexer (WDM); multiplexing the one ormore light pulses from the interrogator unit with the proximal WDM intoan upgoing transmission fiber and a down-going transmission fiber;receiving the one or more light pulses with a distal WDM that isoptically connected to the upgoing transmission fiber and the down-goingtransmission fiber; multiplexing the one or more light pulses from theupgoing transmission fiber and the down-going transmission fiber intoone or more downhole sensing fibers; and receiving backscatter lightfrom at least one of the one or more downhole sensing fibers.
 13. Themethod of claim 12, further comprising generating one or more lightpulses with at least one wavelength or within a bandwidth of wavelengthswith an interrogator disposed in the interrogator unit.
 14. The methodof claim 13, further comprising combining the one or more light pulsesfrom one or more interrogators with the proximal WDM.
 15. The method ofclaim 13, wherein each interrogator is configured to launch light pulsesat different pulse repetition rates.
 16. The method of claim 12, whereina distal circulator is optically connected to the distal WDM, the one ormore downhole sensing fibers, and a second distal WDM.
 17. The method ofclaim 16, wherein the second distal WDM is optically connected to theupgoing transmission fiber and the down-going transmission fiber. 18.The method of claim 17, wherein a proximal circulator is opticallyconnected to the proximal WDM, the upgoing transmission fiber, and thedown-going transmission fiber.
 19. The method of claim 12, furthercomprising optimizing a sampling frequency by: identifying a length ofat least one of the one or more downhole sensing fibers opticallyconnected to the distal WDM; identifying one or more sensing regions onthe at least one of the one or more downhole sensing fibers; andidentifying a minimum time interval that is between an emission of theone or more light pulses such that at no point in time the backscatteredlight arrives back at the interrogator unit during an emission of theone or more light pulses.
 20. The method of claim 12, further comprisingperforming a distributed fiber optic sensing measurement or a discretefiber optic sensing measurement in one or more sensing regions of theone or more downhole sensing fibers.